Fix double unsubscribe from TCPConnection event on non-Windows systems

Closes #4639

Author: SeanTAllen

packages/net/tcp_connection.pony

@@ -636,7 +636,18 @@ actor TCPConnection is AsioEventNotify
             @pony_asio_event_unsubscribe(event)
           end
           @pony_os_socket_close(fd)
-          _try_shutdown()
+          ifdef windows then
+            // Fix for windows happy-eyeballs related hang from
+            // 95a06bb290150d13b5764de70c1332a6d7b237cf.
+            // This should not be called on non-IOCP as that can result in
+            // issue #4639 where we call `pony_asio_event_unsubscribe` more than
+            // once which can result in an actor that is already marked
+            // for destruction getting a message sent to it.
+            // This might be a race condition on Windows (I think it could be)
+            // but Windows is so slow closing down connections that we don't
+            // to ever hit it if it is indeed a race condition.
+            _try_shutdown()
+          end
         end
       else
         // It's not our event.